Jacques Roy

23 – Estimations of Global Terrestrial Productivity: Converging toward a Single Number?

Publisher Summary Terrestrial ecosystems significantly contribute to the global carbon cycle. The chapter discusses that stocks and fluxes are increasingly altered by human activities, through changes in land use, in atmospheric composition, and in climate. This carbon sink results from an increase in global terrestrial net primary productivity (NPP). Thus, it is essential to know what the present value of global NPP is, and whether it will continue to increase to sustain an important terrestrial carbon sink. This chapter uses recent information provided in each of the biome-updated estimates of global values for NPP and phytomass. Finally, the current and expected changes in global NPP and net ecosystem productivity are discussed. The main source of uncertainty on global NPP lies in the biome area, and especially in forest area. Significant progress on biome maps is expected in the near future from the use of satellite data, once the classification done using such data can be given biome names that can receive a large acceptance. There is also some uncertainty on the current and future rates of change in NPP. Research on this aspect, at all scales, is developing fast. The chapter emphasizes on predicting more precisely the impact of human activities on NPP and on the carbon cycle in general. This information is crucial to design and implement sustainable development of human societies.

Plant species diversity as a driver of early succession in abandoned fields: a multi-site approach

Abstract Succession is one of the most studied processes in ecology and succession theory provides strong predictability. However, few attempts have been made to influence the course of succession thereby testing the hypothesis that passing through one stage is essential before entering the next one. At each stage of succession ecosystem processes may be affected by the diversity of species present, but there is little empirical evidence showing that plant species diversity may affect succession. On ex-arable land, a major constraint of vegetation succession is the dominance of perennial early-successional (arable weed) species. Our aim was to change the initial vegetation succession by the direct sowing of later-successional plant species. The hypothesis was tested that a diverse plant species mixture would be more successful in weed suppression than species-poor mixtures. In order to provide a robust test including a wide range of environmental conditions and plant species, experiments were carried out at five sites across Europe. At each site, an identical experiment was set up, albeit that the plant species composition of the sown mixtures differed from site to site. Results of the 2-year study showed that diverse plant species mixtures were more effective at reducing the number of natural colonisers (mainly weeds from the seed bank) than the average low-diversity treatment. However, the effect of the low-diversity treatment depended on the composition of the species mixture. Thus, the effect of enhanced species diversity strongly depended on the species composition of the low-diversity treatments used for comparison. The effects of high-diversity plant species mixtures on weed suppression differed between sites. Low-productivity sites gave the weakest response to the diversity treatments. These differences among sites did not change the general pattern. The present results have implications for understanding biological invasions. It has been hypothesised that alien species are more likely to invade species-poor communities than communities with high diversity. However, our results show that the identity of the local species matters. This may explain, at least partly, controversial results of studies on the relation between local diversity and the probability of being invaded by aliens.

Plant diversity increases soil microbial activity and soil carbon storage

Plant diversity strongly influences ecosystem functions and services, such as soil carbon storage. However, the mechanisms underlying the positive plant diversity effects on soil carbon storage are poorly understood. We explored this relationship using long-term data from a grassland biodiversity experiment (The Jena Experiment) and radiocarbon ((14)C) modelling. Here we show that higher plant diversity increases rhizosphere carbon inputs into the microbial community resulting in both increased microbial activity and carbon storage. Increases in soil carbon were related to the enhanced accumulation of recently fixed carbon in high-diversity plots, while plant diversity had less pronounced effects on the decomposition rate of existing carbon. The present study shows that elevated carbon storage at high plant diversity is a direct function of the soil microbial community, indicating that the increase in carbon storage is mainly limited by the integration of new carbon into soil and less by the decomposition of existing soil carbon.

Functional stability, substrate utilisation and biological indicators of soils following environmental impacts

Abstract Stability of a soil property to perturbation comprises both resistance and resilience. Resistance is defined as the ability of the soil to withstand the immediate effects of perturbation, and resilience the ability of the soil to recover from perturbation. Functional stability is used here to describe the stability of a biological function to perturbation, rather than the stability of physical structure or chemical properties. The function chosen for this study was the short-term decomposition of added plant residues, and the perturbations were copper and heat stresses. Previous studies had shown that functional stability was reduced greatly in soils with experimentally reduced biodiversity. The objective of this study was to determine the relative sensitivity of functional stability and potential indicators of biological status to detect alteration of field soils by various environmental impacts. Functional stability, protozoan populations and substrate mineralisation kinetics, were measured on paired soils with: high or low plant species diversity; hydrocarbon pollution or not; extensive or intensive agricultural management practices. Substrate mineralisation kinetics were poorly related to the soil’s antecedent conditions and were stimulated significantly by hydrocarbon pollution. Protozoan populations were potentially useful for detecting differences within soil type, but will require greater taxonomic input to be most useful. Functional stability, particularly resistance, was able to quantify differences between and within soils. The potential development of the technique in relation to soil health is discussed.

Germination and population dynamics of Cistus species in relation to fire

1. The shrubs Cistus monspeliensis and C. albidus are obligate seeders which are often dominant in Mediterranean habitats degraded by recurrent fires. 2. To determine the role of fire in the regulation of the population dynamics of these species, the germination requirements and population age-structure have been examined for populations in the South of France. 3. Germination of both species was enhanced by a change in light quality (red/farred ratio) similar to that which occurs when fire removes the light-filtering green leaves, but to a lesser extent than by a heat pretreatment designed to simulate fire temperatures

High variation in foliage and leaf litter chemistry among 45 tree species of a neotropical rainforest community

Distinct ecosystem level carbon : nitrogen : phosphorus (C : N : P) stoichiometries in forest foliage have been suggested to reflect ecosystem-scale selection for physiological strategies in plant nutrient use. Here, this hypothesis was explored in a nutrient-poor lowland rainforest in French Guiana. Variation in C, N and P concentrations was evaluated in leaf litter and foliage from neighbour trees of 45 different species, and the litter concentrations of major C fractions were also measured. Litter C ranged from 45.3 to 52.4%, litter N varied threefold (0.68-2.01%), and litter P varied seven-fold (0.009-0.062%) among species. Compared with foliage, mean litter N and P concentrations decreased by 30% and 65%, respectively. Accordingly, the range in mass-based N : P shifted from 14 to 55 in foliage to 26 to 105 in litter. Resorption proficiencies indicated maximum P withdrawal in most species, but with a substantial increase in variation in litter P compared with foliage. These data suggest that constrained ecosystem-level C : N : P ratios do not preclude the evolution of highly diversified strategies of nutrient use and conservation among tropical rainforest tree species. The resulting large variation in litter quality will influence stoichiometric constraints within the decomposer food web, with potentially far-ranging consequences on nutrient dynamics and plant-soil feedbacks.

Soil microbial diversity and soil functioning affect competition among grasses in experimental microcosms

A gradient of microbial diversity in soil was established by inoculating pasteurized soil with microbial populations of different complexity, which were obtained by a combination of soil fumigation and filtering techniques. Four different soil diversity treatments were planted with six different grass species either in monoculture or in polyculture to test how changes of general microbial functions, such as catabolic diversity and nutrient recycling efficiency would affect the performance of the plant communities. Relatively harsh soil treatments were necessary to elicit visible effects on major soil processes such as decomposition and nitrogen cycling due to the high redundancy and resilience of soil microbial communities. The strongest effects of soil diversity manipulations on plant growth occurred in polycultures where interspecific competition between plants was high. In polycultures, soil diversity reduction led to a gradual, linear decline in biomass production of one subordinate grass species (Bromus hordeaceus), which was compensated by increased growth of two intermediate competitors (Aegilops geniculata, B. madritensis). This negative covariance in growth of competing grass species smoothed the effects of soil diversity manipulations at the plant community level. As a result, total shoot biomass production remained constant. Apparently the effects of soil diversity manipulations were buffered because functional redundancy at both, the microbial and the plant community level complemented each other. The results further suggests that small trade-offs in plant fitness due to general functional shifts at the microbial level can be significant for the outcome of competition in plant communities and thus diversity at much larger scales.

Elevated CO2 maintains grassland net carbon uptake under a future heat and drought extreme

Significance Ecosystems are responding to climate change and increasing atmospheric CO2 concentrations. Interactions between these factors have rarely been assessed experimentally during and after extreme climate events despite their predicted increase in intensity and frequency and their negative impact on primary productivity and soil carbon stocks. Here, we document how a grassland exposed to a forecasted 2050s climate shows a remarkable recovery of ecosystem carbon uptake after a severe drought and heat wave, this recovery being amplified under elevated CO2. Over the growing season, elevated CO2 entirely compensated for the negative impact of extreme heat and drought on net carbon uptake. This study highlights the importance of incorporating all interacting factors in the predictions of climate change impacts. Extreme climatic events (ECEs) such as droughts and heat waves are predicted to increase in intensity and frequency and impact the terrestrial carbon balance. However, we lack direct experimental evidence of how the net carbon uptake of ecosystems is affected by ECEs under future elevated atmospheric CO2 concentrations (eCO2). Taking advantage of an advanced controlled environment facility for ecosystem research (Ecotron), we simulated eCO2 and extreme cooccurring heat and drought events as projected for the 2050s and analyzed their effects on the ecosystem-level carbon and water fluxes in a C3 grassland. Our results indicate that eCO2 not only slows down the decline of ecosystem carbon uptake during the ECE but also enhances its recovery after the ECE, as mediated by increases of root growth and plant nitrogen uptake induced by the ECE. These findings indicate that, in the predicted near future climate, eCO2 could mitigate the effects of extreme droughts and heat waves on ecosystem net carbon uptake.

Intraspecific variability of phenolic concentrations and their responses to elevated CO2 in two mediterranean perennial grasses

Abstract Intraspecific variability of total phenolic compound concentrations and their responses to elevated CO 2 were studied in two wild Mediterranean perennial grasses, Dactylis glomerata and Bromus erectus . Ten and nine genotypes of each species were grown in climate-controlled naturally-lit glasshouses under plant intergenotypic and interspecific competition for water, light and nutrients. Carbon source–sink balance hypotheses of resource allocation were also tested. Elevated CO 2 induced changes in dry shoot biomass (DM), leaf total non-structural carbohydrate concentrations [TNC] and leaf nitrogen concentrations [N] found in a previous study (New Phytol. 143 (1999) 73) were related to changes in phenolic compound concentrations. Phenolic compound concentrations increased to 15.2% DM in D. glomerata and 86.9% DM in B. erectus under elevated CO 2 . These changes were more pronounced when expressed on a structural dry mass basis (DM st ). Increases in DM st and [TNC st ] and decreases in [N st ] were also found according to current resource allocation hypotheses. However, there were no proportional changes between phenolic responses to elevated CO 2 and DM st , [TNC st ] and [N st ] responses. Phenolic concentrations were highly determined by genetics in both species, but all studied genotypes responded in a similar way to elevated CO 2 . Considering the present experimental conditions with plants growing in intraspecific and interspecific competition, the absence of CO 2 ×genotype interaction would lead to little changes of fitness in terms of antiherbivore chemical defence, and, therefore, to low evolutionary consequences in CBSC under the increasing atmospheric CO 2 concentrations of the next decades.

Processes driving nocturnal transpiration and implications for estimating land evapotranspiration

Evapotranspiration is a major component of the water cycle, yet only daytime transpiration is currently considered in Earth system and agricultural sciences. This contrasts with physiological studies where 25% or more of water losses have been reported to occur occurring overnight at leaf and plant scales. This gap probably arose from limitations in techniques to measure nocturnal water fluxes at ecosystem scales, a gap we bridge here by using lysimeters under controlled environmental conditions. The magnitude of the nocturnal water losses (12–23% of daytime water losses) in row-crop monocultures of bean (annual herb) and cotton (woody shrub) would be globally an order of magnitude higher than documented responses of global evapotranspiration to climate change (51–98 vs. 7–8 mm yr−1). Contrary to daytime responses and to conventional wisdom, nocturnal transpiration was not affected by previous radiation loads or carbon uptake, and showed a temporal pattern independent of vapour pressure deficit or temperature, because of endogenous controls on stomatal conductance via circadian regulation. Our results have important implications from large-scale ecosystem modelling to crop production: homeostatic water losses justify simple empirical predictive functions, and circadian controls show a fine-tune control that minimizes water loss while potentially increasing posterior carbon uptake.